When growing different structures bq LPE it is necessary to know exactly the temperature on the boundary between the liquid and solid phases. That temperature can not be measured directly by a thermocouple because normally it is located at some distance From this boundary and in any LPE system there are always unknown temperature gradients. However the interface temperature and its distribution along the LPE boat can be determined by solubility measurements. The accuracy of such measurements will be affected by several factors and phenomena. One of such effects is related to the evaporation of volatile components. In this work we have used In-P liquid solutions and its phase diagram to determine the temperature distribution along an LPE boat and have also studied the effect of phosphorus evaporation. It is shown that phosphorus evaporation can be neglected for temperatures under 650 degrees C but at higher temperatures it is significant.